The present invention concerns a bone screw assembly, and particularly an assembly useful for engagement in the vertebrae of the spine. The invention contemplates an assembly that is capable of supporting a bone engaging fastener at multiple angular orientations with respect to an elongated member extending along the spine.
Several techniques and systems have been developed for correcting and stabilizing the spine and for facilitating fusion at various levels of the spine. In one type of system, a bendable rod is disposed longitudinally along the length of the spine or vertebral column. The rod is preferably bent to correspond to the normal curvature of the spine in the particular region being instrumented. For example, the rod can be bent to form a normal kyphotic curvature for the thoracic region of the spine, or a lordotic curvature for the lumbar region. In accordance with such a system, the rod is engaged to various vertebrae along the length of the spinal column by way of a number of fixation elements. A variety of fixation elements can be provided which are configured to engage specific portions of the vertebra. For instance, one such fixation element is a hook that is configured to engage the laminae of the vertebra. Another very prevalent fixation element is a spinal screw which can be threaded into various aspects of the vertebral bone.
In one typical procedure utilizing a bendable rod, the rod is situated on opposite sides of the spine or spinous processes. A plurality of bone screws are threaded into a portion of several vertebral bodies, very frequently into the pedicles of these vertebrae. The rods are affixed to these plurality of bone screws to apply corrective and stabilizing forces to the spine.
One example of a rod-type spinal fixation system is the TSRH.RTM. Spinal System sold by Danek Medical, Inc. The TSRH.RTM. System includes elongated rods and a variety of hooks, screws and bolts all configured to create a segmental construct throughout the spine. In one aspect of the TSRH.RTM. System, the spinal rod is connected to the various vertebral fixation elements by way of an eyebolt. In this configuration, the fixation elements are engaged to the spinal rod laterally adjacent to the rod. In another aspect of the TSRH.RTM. System, a variable angle screw is engaged to the spinal rod by way of an eyebolt. The variable angle screw allows pivoting of the bone screw in a single plane that is parallel to the plane of the spinal rod. Details of this variable angle screw can be found in U.S. Pat. No. 5,261,909 to Sutterlin et al., owned by the Assignee of the present invention. One goal achieved by the TSRH.RTM. System is that the surgeon can apply vertebral fixation elements, such as a spinal hook or a bone screw, to the spine in appropriate anatomic positions. The TSRH.RTM. System also allows the surgeon to easily engage a bent spinal rod to each of the fixation elements for final tightening.
Another rod-type fixation system is the Cotrel-Dubosset/CD Spinal System sold by Sofamor Danek Group, Inc. Like the TSRH.RTM. System, the CD.RTM. System provides a variety of fixation elements for engagement between an elongated rod and the spine. In one aspect of the CD.RTM. System, the fixation elements themselves include a body that defines a slot within which the spinal rod is received. The slot includes a threaded bore into which a threaded plug is engaged to clamp the rod within the body of the fixation element. The CD.RTM. System includes hooks and bone screws with this "open-back" configuration. Details of this technology can be found in U.S. Pat. No. 5,005,562 to Dr. Cotrel. One benefit of this feature of the CD.RTM. System is that the fixation element is positioned directly beneath the elongated rod. This helps reduce the overall bulkiness of the implant construct and minimizes the trauma to surrounding tissue.
On the other hand, these fixation elements of the CD.RTM. System are capable only of pivoting about the spinal rod to achieve variable angular positions relative to the rod. While this limited range of relative angular positioning is acceptable for many spinal pathologies, many other cases require more creative orientation of a bone screw, for instance, relative to a spinal rod. Certain aspects of this problem are addressed by the variable angle screw of the TSRH.RTM. System, as discussed in the '909 Patent. However, there is a need for a bone screw that is capable of angular orientation in multiple planes relative to the spinal rod. Preferably, the bone screw is capable of various three-dimensional orientations with respect to the spinal rod. Screws of this type have been referred to as poly-axial or multi-axial bone screws.
Others have approached the solution to this problem with various poly-axial screw designs. For example, in U.S. Pat. No. 5,466,237 to Byrd et al., a bone screw is described which includes a spherical projection on the top of the bone screw. An externally threaded receiver member supports the bone screw and a spinal rod on top of the spherical projection. An outer nut is tightened onto the receiver member to press the spinal rod against the spherical projection to accommodate various angular orientations of the bone screw relative to the rod. While this particular approach utilizes a minimum of components, the security of the fixation of the bone screw to the rod is lacking. In other words, the engagement or fixation between the small spherical projection on the bone screw and the spinal rod is readily disrupted when the instrumentation is subjected to the high loads of the spine, particularly in the lumbar region.
The same inventors implemented a somewhat different approach in U.S. Pat. No. 5,474,555. In this patent, an anchor receives a semi-spherical head of a bone screw within a recess. The anchor includes a rod channel transverse to the screw recess. The anchor is externally threaded to receive an internally threaded nut. In one embodiment, the nut is threaded directly down onto the spinal rod to clamp the rod within the channel. In another embodiment, a cap is provided between the rod and nut. In both embodiments, neither the rod nor the external not impart any clamping force onto the head of the bone screw.
In another approach shown in U.S. Pat. No. 4,946,458 to Harms et al., a spherical headed bone screw is supported within separate halves of a receiver member. The bottom of the halves are held together by a retaining ring. The top of the receiver halves are compressed about the bone screw by nuts threaded onto a threaded spinal rod. In another approach taken by Harms et al., in U.S. Pat. No. 5,207,678, a receiver member is flexibly connected about a partially spherical head of a bone screw. Conical nuts on opposite sides of the receiver member are threaded onto a threaded rod passing through the receiver. As the conical nuts are threaded toward each other, the receiver member flexibly compresses around the head of the bone screw to clamp the bone screw in its variable angular position. One detriment of the systems in the two Harms et al. patents is that the spinal rod must be threaded in order to accept the compression nuts. It is known that threaded rods can tend to weaken the rods in the face of severe spinal loads. Moreover, the design of the bone screws in the '458 and '678 Patents require a multiplicity of parts and are fairly complicated to achieve complete fixation of the bone screw.
Two patents to Errico et al., U.S. Pat. Nos. 5,549,608 and 5,554,157, depict two alternative approaches to multi-angle bone screw apparatus. The '608 Patent describes a complicated array of components that includes a tapered lower portion defining a spherical recess to receive the head of a bone screw. The lower portion is slotted so that the lower portion can be compressed about the bone screw head by operation of a ring pushed down the tapered lower portion. A hollow cylindrical rod securing sleeve fits over an upper portion of the coupling element that is operable to clamp the rod within the coupling element as the sleeve is pushed down onto the ring. The coupling element includes a threaded post onto which a nut is threaded that is operable to push the sleeve down onto the rod and onto the ring to compress the lower portion against the bone screw head. Like the Harms devices described above, the approach in the '608 Patent involves a multiplicity of parts and excessive "fiddle factor" for use in a spinal surgery.
The second alternative in the Errico '157 Patent is similar to the approach taken in the Puno '555 Patent, except that the rod contacts the spherical head of the bone screw in the '157 Patent. An external nut is threaded directly down onto the spinal rod which then bears directly on the bone screw head to clamp the head within a spherical recess. While this design offers much greater simplicity than the Errico '608 Patent it suffers from the point contact between the rod and the spherical head of the bone screw. It is uncertain whether this clamping mechanism is sufficient to maintain the relative position between bone screw and spinal rod under severe spinal loads.
In recent years, a special material known as "shape-memory alloy" has found its way into the field of medical devices. These materials are alloys of known metals, such as copper and zinc, nickel and titanium, silver and cadmium, and others, that are known to exhibit a "shape-memory" in which a particular component formed of a shape-memory alloy (SMA) will change shape upon changes in temperature.
The shape-memory characteristics of SMAs occur when the alloy changes from a martensitic crystal phase to an austenitic crystal phase. In the martensitic stage, the SMA is relatively weak and pliable. As the temperature of the SMA component is increased above a transformation temperature range, the SMA transforms to its austenitic phase in which the material is relatively strong with super-elastic properties. Generally, the strength and super-elastic characteristics of an SMA tend to increase toward the high temperature end of the transformation temperature range and decrease toward the low temperature end.
In use, an object made of an SMA is formed into a particular shape at a temperature that is either above or below the transformation temperature range. The object will then change shape as its temperature increases or decreases through that transformation temperature range. In the field of medical devices, a device is formed into its in situ shape at the high temperature, or the temperature above the transformation temperature range. The device is then cooled to be provided to the surgeon for implantation. One known application of SMA technology in the medical field is a vena cava filter that assumes a smaller shape at its low temperature, and then expands to its larger shape within a blood vessel when heated to body temperature.
While there are many alloys that exhibit shape memory characteristics, one of the more common SMAs in the medical field is an alloy of nickel and titanium. One such well known alloy is Nitinol.RTM., which has proven highly effective for devices placed within the human body because its transformation temperature range falls between room temperature and normal human body temperature. Shape-memory technology has also found its way into the field of orthodontics, as described in U.S. Pat. No. 5,551,871 to Besselink et al. This patent describes face bow head gear used for aligning teeth of a patient in which an SMA wire is used to align and hold the teeth of a patient. The '871 Patent also discloses devices for use in treating scoliosis including a transverse connector and bone engaging fasteners that clamp to an elongated rod by way of shape-memory characteristics. While the '871 Patent shows one use of SMA technology for spinal hooks and screws, the hooks and screws are only capable of pivoting about the axis of the rod. In this regard, the devices shown in the '871 Patent are similar to the components of the CD.RTM. System discussed above.
There is a need remaining in the industry for a multi-axial or poly-axial bone screw that can be readily and securely engaged to an elongated spinal rod. Preferably, the spinal rod can be of any configuration--i.e., smooth, roughened, knurled or even threaded.
This need also encompasses the goal of minimizing the profile and bulk of any of the components used to engage the bone screw to the spinal rod in a variety of angular orientations. Moreover, it is desirable to reduce the number of components of the system that must be manipulated by the surgeon during a surgical procedure.